Hi ls13coco,
I own the Sundara, Arya and HE6se. I think the answer to your question would depend on the kind of music you listen to (e.g. metal and hardrock vs classical or ambient) or on how you want it to sound (more relaxed or not).
My dealer once explained to me that Hifiman has been developing planar magnetic headphones along two different main lines, roughly said the round ones and the elongated ones. I think it makes sense, so that in the "round line" the logical upgrade for the Sundara would be the HE6se (and ultimately the Susvara) while in the other line the Arya would be an upgrade instead for the Edition X xx and the Ananda. Going from the Sundara to the Arya would then also mean changing lines.
Changing lines doesn't make the comparison so easy as the lines seem to aim at different genres. The round line would be rougher (esp. the HE6) and tend to be more optimized for genres like metal and hard-rock. The elongated line would be smoother and tend to be more optimized for classical music or softer genres. When you reach the current TOTL models like the Susvara and the HE1000se the differences between the lines are less perceptible.
Back to Sundara vs Arya.
Re. most sonic aspects, the Arya clearly beats the Sundara, e.g. in resolution, speed, cleanliness (of the background, what SNMR would measure), soundstage, imaging... I think everyone would agree with this.
And yet... the technical superiority isn't all, their character according to their differing lines comes into play.
For example if I listen to Amon Amarth (Viking melodic death metal, e.g. their latest album Jomsviking), the Sundara rocks better for me, gives me a strong temptation of turning the volume up (I have to fight to resist this temptation, as I try to listen at low levels to preserve my ears) and of banging my head (I don't headbang but I feel the temptation strongly). If I listen to the very same music with the Arya, I don't experience these strong feelings, the music feels more controlled, "behaved" and less exciting with such genres.
So in my experience, if you like your Sundara for having fun (rocking) with metal and hardrock, the real upgrade for the Sundara is the HE6se, it is somehow "wild" and "harsh" in a positive sense (i.e. in the context of metal music), not as great as the Susvara but so much worthier for the price you have to pay for it.
If you listen mainly to classical or ambient music, then the Arya is a definite upgrade for the Sundara, as it will be better to "change lines".
If you want a more relaxed presentation of metal (e.g. to focus on your work while listening), then the Arya will be more suitable but would you then need to pay more money if you won't focus on the more detailed presentation?
I personally bought the Arya for listening to dark ambient, at which I find it excels, and to some bands which mix ambient with metal (e.g. Elffor) or mix metal with neoclassical, neofolk or rock.
In general I think that the Arya is a bargain compared to the HE1000xx, while the Sundara at € 350 is much more than a bargain, it is a real steal, i.e. the Arya is great for the price you pay for it but not incredibly great for the price like the Sundara.
Hoping this may help you,
bidn